Heterojunction bipolar transistors (HBTs) exhibit desirable features such as high current gain and an extremely high cut-off frequency for switching applications, and high power gain and power density for microwave amplifier applications. Even so, as with other types of semiconductor devices there is demand for ever higher operating frequencies or switching speeds from HBTs. Efforts to accomplish this increased performance invariably lead to a scaling down of transistor size. However, as the emitter in an HBT is scaled down, the current gain of the transistor is also dramatically reduced. This effect threatens to limit the level of integration and circuit complexity that can be realized with HBT technology, and has implications for the reliability of HBT power transistors as well.
The reduction in current gain is related to the ratio of the HBT's emitter perimeter-to-area ratio. A cross-sectional diagram of a typical npn HBT is shown in FIG. 1 (the base layer thickness relative to the other layers is highly exaggerated). In operation, a flow of electrons is established from the emitter, through the base, and into the collector. This electron current is modulated by holes injected into the base from the base contacts. These holes recombine with some of the electrons from the emitter and therefore result in finite current gain. One limitation on current gain is the high density of carrier traps which exists at an exposed semiconductor surface. The trap density is typically large enough to create an electric field near the surface that extends some distance into the base layer. Electrons injected near the edge of the emitter mesa are drawn to the surface of the base layer by this electric field where they recombine in the abundance of traps present at the surface. Hence, the total minority carrier current from the emitter has a desirable component, i.e. the carriers that transit the base to the collector; and an undesirable component, i.e. the carriers that recombine at the surface of the base layer. Unfortunately, the desirable current scales with the area of the emitter, while the undesirable current scales with the perimeter. Consequently, as the emitter dimensions are reduced, the perimeter current becomes a larger percentage of the total emitter current. This results in a decrease of the current gain of the transistor.
Past efforts at solving the problem of surface recombination at the extrinsic base surface have included physical and chemical passivation treatments. Sputtered SiN, depleted AlGaAs passivation ledges, and sulfide-base coatings have been reported. See O. Nakajima, et al., "Emitter-Base Junction Size Effect on Current Gain H.sub.fe of AlGaAs/GaAs Heterojunction Bipolar Transistors", Japanese Journal of Applied Physics, Vol. 24, No. 8, pp. L596-L598, August 1985; R. J. Malik, et al., "Submicron Scaling of AlGaAs/GaAs Self-aligned Thin Emitter Heterojunction Bipolar Transistors with Current Gain Independent of Emitter Area", Electronics Letters, Vol. 25, No. 17, pp. 1175-1177, Aug. 17, 1989; S. Tiwari, et al., "Surface Recombination in AlGaAs/GaAs Heterostructure Bipolar Transistors", Journal of Applied Physics, Vol. 64, No. 10, pp. 5009-5012, Nov. 15, 1988. However, these solutions have drawbacks such as process complexity and performance shortcomings that prevent them from offering a complete answer to the problem of extrinsic base surface recombination. For example, depleted AlGaAs passivation ledges, shown in FIG. 2, while effective in reducing the effects of surface states, require that the spacing between the base contact 10 and the active emitter 12 be large enough to accommodate a passivation ledge 14 extending between the active emitter 12 and the base contact 10. High frequency operation demands a lower base resistance and base-collector junction capacitance than is generally possible with such a technique. In addition, the passivation ledge structure does not lend itself to the self-aligned fabrication techniques necessary for economical volume production.
Another prior art approach to passivating the base surface is shown in FIG. 3 and is described in Malik, supra at 1176. It is a modification of the ledge passivation approach. The surface of the GaAs base layer 20 is completely covered by a thin AlGaAs emitter layer 22, in contrast to the ledge structure described above. As in the ledge approach the thin AlGaAs layer 22 extending between the emitter mesa 24 and the base contacts 26 is fully depleted. It serves to passivate the surface states at the surface of the base layer and therefore minimizes surface recombination. In this particular prior art structure, the base layer is primarily GaAs, but contains a small mole fraction of aluminum. The aluminum content is graded from 0% at the base-collector interface to 6% at the base-emitter interface. This sets up a quasi-electric field that helps to keep the minority carriers from migrating to the surface to recombine. However, a problem with this approach is that the base contacts are formed on the emitter layer. Metal from the contacts spikes through the emitter layer and into the base layer upon being alloyed. Since the base layer is typically very thin (approximately 600 .ANG.), it is difficult to alloy the contacts such that metal 27 extends into the base layer 20 without also extending into the underlying collector layer 28. A structure that relies on such alloyed contacts suffers from process uncertainty and has been shown to be unreliable in production. The present invention is intended to address the reliability problems of this prior art structure and the process limitations of the ledge structure.